Color digital imaging systems, such as digital cameras, typically employ a single image sensor, either a charged coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, to digitally capture a scene of interest. Image sensors typically include an array of tightly spaced optical detectors, such as photodiodes, that generate an electrical response in proportion to the intensity of incident light. The dynamic range of the individual optical detectors is defined by the minimum amount of light that is required to generate an electrical response on the low end and the maximum amount of light beyond which the electrical response of the optical detector does not change (the saturation point) on the high end.
The dynamic range of an image sensor is an important characteristic when capturing high contrast images. When the bright and/or dark areas of an image exceed the dynamic range of the image sensor, the quality of the captured image suffers. If the sensitivity of the image sensor is adjusted (i.e., by decreasing the exposure time) to sufficiently capture the features of bright areas in an image, then the features of the dark areas are not captured. Likewise, if the sensitivity of the image sensor is adjusted (i.e., by increasing the exposure time) to sufficiently capture the features of dark areas in an image, then the features of the bright areas are not sufficiently captured.
One known technique for capturing high contrast scenes with a digital image sensor involves capturing two images of the same scene in rapid succession, with the sensitivity of the image sensor set to capture the bright areas in a first image and the dark areas in a second image. The two images are then used to generate a composite image that includes features in both the bright and dark areas. FIG. 1 depicts an example of a two-image technique in which the features of bright areas (i.e., background scenery) are captured in a first image 102 using a first exposure time and the features of dark areas (i.e., a person in the foreground) are captured in a second image 104 using a second exposure time. Features of the bright areas from the first image and features of the dark areas from the second image are then used to generate a combined image 106 that includes features of both the bright and dark areas.
Although the two-image technique can extend the dynamic range of an image sensor, the technique requires enough memory to store two images before the combined image is generated and enough processing power to generate the combined image. In addition to the memory and processing requirements of the two-image technique, any changes in the scene that occur between the capturing of the first and second images will degrade the quality of the combined image.
In view of the problems involved with capturing high contrast color images with digital imaging systems, what is needed is a system and method for extending the dynamic range of digital imaging systems.